Re: [RFC][PATCH] kprobes/x86: While list ftrace locations in kprobe blacklist areas

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 11 Oct 2019 17:39:29 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Hi Steve,
> 
> On Thu, 10 Oct 2019 17:52:16 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > From: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > 
> > I noticed some of my old tests failing on kprobes, and realized that
> > this was due to black listing irq_entry functions on x86 from being
> > used by kprobes. IIRC, this was due to the cr2 being corrupted and
> > such, and I believe other things were to cause. But black listing all
> > irq_entry code is a big hammer to this.  
> 
> OK, I think if we can use ftrace for hooking, probing on "that address"
> is good, but the function body still can not be probed.
> 
> > 
> >  (See commit 0eae81dc9f026 "x86/kprobes: Prohibit probing on IRQ
> >  handlers directly" for more details)
> > 
> > Anyway, if kprobes is using ftrace as a hook, there shouldn't be any
> > problems here. If we white list ftrace locations in the range of
> > kprobe_add_area_blacklist(), it should be safe.  
> 
> Agreed.
> 
> > 
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > ---
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index d9770a5393c8..9d28a279282c 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -2124,6 +2124,11 @@ int kprobe_add_area_blacklist(unsigned long start, unsigned long end)
> >  	int ret = 0;
> >  
> >  	for (entry = start; entry < end; entry += ret) {
> > +#ifdef CONFIG_KPROBES_ON_FTRACE
> > +		/* We are safe if using ftrace */
> > +		if (ftrace_location(entry))
> > +			continue;
> > +#endif  
> 
> Have you tested the patch? it doesn't measure the entry function's size.
> (kprobe_add_ksym_blacklist(entry) returns the function size)

Yeah, I tested the patch, but it is obviously buggy. It fixed the issue
I was having (was able to test against do_IRQ ;-)

> 
> Could you do this in kprobe_add_ksym_blacklist()?
> Instead of continue, increment ent->start_addr by MCOUNT size(?) will be OK.
> (Note that since each blacklist symbol is managed independently, you can make
>  a "gap" between them as a safe area)

OK, I'll move it to kprobe_add_ksym_blacklist.

-- Steve

> 
> Thank you,
> 
> >  		ret = kprobe_add_ksym_blacklist(entry);
> >  		if (ret < 0)
> >  			return ret;  
> 
>